Fischer-tropsch catalyst comprising cobalt, magnesium and precious metal

ABSTRACT

A method is described for preparing a catalyst precursor suitable for use in the Fischer-Tropsch synthesis of hydrocarbons including 10 to 40% by weight of cobalt oxide crystallites and 0.05 to 0.5% by weight of a precious metal promoter, dispersed over the surface of a porous transition alumina wherein the surface of the transition alumina has been modified by inclusion of 0.25 to 3.5% wt magnesium, including the steps of: 
     (a) forming a modified catalyst support by impregnating a transition alumina with a magnesium compound, drying and calcining the impregnated alumina in a first calcination at a temperature 600° C. to convert the magnesium compound into oxidic form, and
 
(b) forming a catalyst precursor by impregnating the modified catalyst support with a cobalt compound and precious metal promoter compound, drying and calcining the impregnated catalyst support in a second calcination to convert the cobalt compound to cobalt oxide.

FIELD OF THE INVENTION

This invention relates to cobalt catalysts and in particular preciousmetal promoted cobalt catalysts supported on a modified transitionalumina, suitable for use in the Fischer-Tropsch synthesis ofhydrocarbons at high temperature.

BACKGROUND OF THE INVENTION

Precious metal-promoted cobalt Fischer-Tropsch catalysts supported ontitania, alumina or silica are known. U.S. Pat. No. 4,088,671 disclosesa hydrocarbon synthesis process using Ru-promoted Co catalysts onvarious supports. U.S. Pat. No. 4,493,905 discloses fluidized bedcatalysts suitable for the Fischer-Tropsch reaction prepared bycontacting finely divided alumina with an aqueous impregnation solutionof a cobalt salt, drying the impregnated support and thereaftercontacting the support with a nonaqueous, organic impregnation solutionof salts of ruthenium and a Group IIIB or IVB metal. U.S. Pat. No.4,822,824 discloses Ru-promoted Co catalysts on titania. U.S. Pat. No.5,302,622 discloses a process for the synthesis of hydrocarbons, using acatalyst comprising cobalt, copper and ruthenium on silica or alumina.The catalysts may be provided as oxidic precursors and reduced to theiractive form in situ in the Fischer-Tropsch reactor, or a pre-reducedcatalyst, with the elemental cobalt either passivated or encapsulated ina wax may be provided to the reactor.

A recurring problem with these catalysts is the rapid deactivation athigh temperatures in use, in particular in operation at temperatures≧230° C.

WO2005/072866 describes a method of producing an alumina-supportedcatalyst, which comprises the following steps: a first impregnation stepin which an initial alumina support material is impregnated with asource of a 2-valent metal capable of forming a spinel compound withalumina; a first calcination step in which the impregnated aluminasupport material is calcined at a temperature of at least 550° C. toproduce a modified alumina support material; a second impregnation stepin which the modified alumina support material is impregnated with asource of catalytically active metal; and a second calcination step inwhich the impregnated modified support material is calcined at atemperature of at least 150° C. The catalytically active metal may becobalt; the source of 2-valent metal may comprises a source of cobalt,zinc, magnesium, manganese, nickel or iron, and a promoter comprisingplatinum, iridium, ruthenium or rhenium may be present. However, thefocus of this disclosure is in improving attrition resistance and themagnesium-containing materials tested contained 5 or 10% magnesium, andwere calcined a temperatures where the magnesium reacts to formmagnesium aluminate. All of the magnesium-containing catalysts showedpoor relative activity.

Similarly, U.S. Pat. No. 7,071,239 discloses Fischer-Tropsch processesand catalysts using stabilised supports based on boehmite which havebeen calcined at high temperature. Preferred structural stabilisers caninclude an element such as cobalt, magnesium, zirconium, boron,aluminium, barium, silicon, lanthanum, oxides thereof or combinationsthereof, or can include precipitated oxides such as co-precipitatedsilica-alumina.

SUMMARY OF THE INVENTION

We have found that with a combination of precious metal promoter andmagnesium-modified alumina support in which the magnesium is present atlow levels and prepared at lower temperatures, the resulting cobaltFischer Tropsch catalysts have increased activity and stability in theFischer Tropsch reaction, particularly at high temperature.

Accordingly the invention provides a method for preparing a catalystprecursor suitable for use in the Fischer-Tropsch synthesis ofhydrocarbons comprising 10 to 40% by weight of cobalt oxide crystallitesand 0.05 to 0.5% by weight of a precious metal promoter, dispersed overthe surface of a porous transition alumina wherein the surface of thetransition alumina has been modified by inclusion of 0.25 to 3.5% wtmagnesium, comprising the steps of:

-   -   (a) forming a modified catalyst support by impregnating a        transition alumina with a magnesium compound, drying and        calcining the impregnated alumina in a first calcination at a        temperature ≦600° C. to convert the magnesium compound into        oxidic form, and    -   (b) forming a catalyst precursor by impregnating the modified        catalyst support with a cobalt compound and precious metal        promoter compound, drying and calcining the impregnated catalyst        support in a second calcination to convert the cobalt compound        to cobalt oxide.

The invention further provides a method for preparing a catalystcomprising the step of reducing the catalyst precursor.

The invention includes therefore the catalyst and catalyst precursorsobtainable by these methods.

The invention further provides a process for the Fischer-Tropschsynthesis of hydrocarbons comprising the step of contacting a synthesisgas mixture comprising hydrogen and carbon monoxide with the catalyst ina Fischer-Tropsch reactor.

The magnesium content of the catalyst precursor is in the range 0.25 to3.5% by weight, preferably about 1.0 to 3.0% by weight, most preferablyabout 1.5 to 2.5% by weight. Higher levels, e.g. about 4% by weight,were found to have a severe deactivating effect on the activity of theresulting catalyst. Due to the relatively low calcination temperatureused, at least a portion of the magnesium in the catalyst precursor ispresent as magnesium oxide (magnesia), MgO. XRD analysis of theMg-modified alumina showed no magnesium aluminate to be present. As theamount of magnesium used to modify the alumina is relatively low, onlythe surface of the alumina is modified so that the bulk properties ofthe alumina remain largely unchanged. The magnesium may be presentwithin the pores of and on the exterior surface of the transitionalumina.

The cobalt content of the catalyst precursor may be in the range 10 to40% by weight, preferably 15 to 30% by weight to keep the number ofimpregnations down during manufacture. The promoter metal may beselected from one or more of Pt, Pd, Re, Ru, Ir or Au, however, Ru isparticularly preferred. Whereas the promoter may be present in an amountin the range 0.05 to 0.5% by weight, the optimal amount of promoter hasbeen found to be in the range 0.05 to 0.25% wt, preferably 0.05 to 0.20%wt, which is considerably lower than in many of the catalysts previouslytested. Lower promoter levels clearly have beneficial handling and costimplications.

The amount of cobalt, precious metal promoter and magnesium in thecatalyst precursor may be readily determined using known methods, e.g.ICP-Atomic Emission Spectroscopy (ICP-AES) or X-Ray Fluorescence (XRF).

The transition alumina may be of the gamma-alumina group, for example aeta-alumina or chi-alumina. These materials may be formed by calcinationof aluminium hydroxides at 400 to 750° C. and generally have a BETsurface area in the range 120 to 400 m²/g. Alternatively, the transitionalumina may be of the delta-alumina group, which includes the hightemperature forms such as delta- and theta- aluminas that may be formedby heating a gamma group alumina to a temperature above about 800° C.The delta-group aluminas generally have a BET surface area in the range50 to 150 m²/g. In the present invention, the transition aluminapreferably comprises gamma alumina and/or a delta alumina with a BETsurface area in the range 120-170 m²/g. Where the catalyst precursor isprepared using a gamma alumina, it is possible by the calcination andreduction procedure to convert at least a portion of this to deltaalumina. Thus the catalyst precursor may be prepared with a gammaalumina yet the catalyst comprise precious-metal promoted cobaltcrystallites dispersed over a gamma alumina, a delta alumina or a mixedphase material comprising delta and gamma aluminas. The alumina shouldbe of suitable purity for use as a catalyst support. In particular thelevel of alkali metal, notably sodium, in the alumina is desirably <50ppm, more preferably <10 ppm. It will be understood that the transitionaluminas used in the present invention are very different in propertiesand behaviour to the hydrated aluminas such as alumina trihydrate andboehmite.

A suitable alumina powder for the catalyst support generally has avolume-median diameter D[ν,0.5] in the range 1 to 200 μm. In certainapplications such as for catalysts intended for use in slurry reactions,it is advantageous to use very fine particles which have a volume-mediandiameter D[ν,0.5], in the range from 1 to 30 μm, e.g. 5 to 25 μm. Forother applications e.g. as a catalyst for reactions carried out in afluidised bed, it may be desirable to use larger particle sizes,preferably in the range 50 to 150 μm. The term volume-median diameterD[ν,0.5], sometimes given as D₅₀ or D_(0.5), may be calculated from theparticle size analysis which may conveniently be effected by laserdiffraction for example using a Malvern Mastersizer.

The pore volume of the alumina support is preferably relatively high inorder to take the cobalt loadings. The pore volume of the alumina ispreferably above 0.30 cm³/g, more preferably in the range 0.35 to 0.85cm³/g, and may be determined by nitrogen physisorption using knowntechniques. It is preferred that the alumina support has a relativelylarge average pore diameter as the use of such supports may givecatalysts of particularly good selectivity. Preferred supports have anaverage pore diameter (APD) of at least 10 nm, particularly in the range12 to 25 nm. [By the term average pore diameter we mean 4 times the porevolume as measured from the adsorption branch of the nitrogenphysisorption isotherm at 0.99 relative pressure divided by the BETsurface area].

The transition alumina is desirably in powder form but may also be ashaped pellet or extrudate.

In powder form, e.g. as a spray-dried powder, the resulting catalystprecursor may be used in slurry-phase Fischer-Tropsch reactors. Acatalyst precursor powder may also be shaped into pellets or extrudates,or used to prepare a wash-coat suitable for coating metal or ceramicsupport structures. In shaped form, e.g. pellets, or extrudates, thecatalyst precursor may be suitable for use in fixed bed Fischer-Tropschreactors. In coated form, e.g. as a washcoating on a metal or ceramicsupport structure, the catalyst may be used in microchannel reactors.

In the activated catalyst, at least a portion of the cobalt oxide(Co₃O₄) in the catalyst precursor is reduced to elemental cobalt. Thecatalyst, when in the reduced state comprise cobalt crystallites thatdesirably have an average size in the range 6 to 14 nm, preferably 6 to10 nm. This may be determined by XRD analysis or from the cobalt surfacearea measurement, which may be suitably determined by hydrogenchemisorption.

Catalysts in the reduced state can be difficult to handle as they canreact spontaneously with oxygen in air, which can lead to undesirableself-heating and loss of activity. Consequently the reduced catalyst ispreferably protected by encapsulation of the reduced catalyst particleswith a suitable barrier coating. In the case of a Fischer-Tropschcatalyst, this may suitably be a hydrocarbon wax. The catalyst may thembe provided in the form of a pellet, pastille or flake according toknown methods. Alternatively the catalyst may be provided as a slurry inmolten wax.

The method for preparing the catalyst support comprises; (a) forming amodified catalyst support by impregnating a transition alumina with amagnesium compound, drying and calcining the impregnated alumina in afirst calcination at a temperature 600° C. to convert the magnesiumcompound into oxidic form, and; (b) forming a catalyst precursor byimpregnating the modified catalyst support with a cobalt compound andprecious metal promoter compound, drying and calcining the impregnatedcatalyst support in a second calcination to convert the cobalt compoundto cobalt oxide.

In impregnation methods, a suitable soluble metal compound, for examplethe metal nitrate or acetate may be impregnated onto a support materialfrom an aqueous or non-aqueous solution, e.g. ethanol, which may includeother materials, and then dried to remove the solvent or solvents. Oneor more soluble metal compounds may be present in the solution. One ormore impregnation steps may be performed to increase metal loading.Impregnation may be performed using any of the methods known to thoseskilled in the art of catalyst manufacture, but preferably is by way ofa so-called ‘dry’ or ‘incipient-wetness’ impregnation as this minimisesthe quantity of solvent used and to be removed in drying. Incipientwetness impregnation comprises mixing the support material with onlysufficient solution to fill the pores of the support. In the presentinvention, amounts up to 150% of incipient wetness volume are preferred.

The magnesium compound may be any suitably soluble magnesium compoundbut is preferably one that converts to magnesia relatively easily uponheating and which does not leave residues that might cause poisoning orundesirable side reactions in the Fischer-Tropsch process. Aparticularly preferred magnesium compound is magnesium nitrate, which isdesirably applied as an aqueous solution to the transition alumina. Asingle impregnation is generally sufficient to provide the desiredmagnesium content in the calcined catalyst precursor.

The impregnated alumina is dried under air or an inert gas such asnitrogen if desired. Drying, to remove solvent, may be done at ambienttemperature from about 20° C., but is preferably performed attemperatures in the range 90-120° C. for 1-8 hours. Vaccuum drying mayalso be used. Alternatively the drying step may be done as the initialpart of the calcination process applied to the impregnated alumina.

The first calcination should be done ≦600° C., and preferably ≦550° C.or even ≦540° C., so that the magnesia formed upon decomposition of themagnesium compound is not converted to the magnesium aluminate spinel.Although calcination may be performed under an inert gas such asnitrogen, it is preferably performed under air. The first calcination isdesirably performed at a temperature ≧250° C., preferably ≧350° C., mostpreferably ≧450° C. so that the conversion of the magnesium compound tomagnesia is essentially completed and is not overly-lengthy. Typicallythe first calcination may be performed by increasing the temperatureover a period of 1-6 hours to a maximum temperature and holding therefor a period up to about 6 hours.

The magnesium-modified alumina may then be treated with the cobalt andprecious metal compounds. Impregnation methods for producing cobaltcatalysts generally comprise combining a catalyst support with asolution of cobalt acetate and/or cobalt nitrate, e.g. cobalt (II)nitrate hexahydrate at a suitable concentration. Whereas a number ofsolvents may be used such as water, alcohols, ketones or mixtures ofthese, preferably the modified support is impregnated using an aqueoussolution of cobalt nitrate. With cobalt nitrate hexahydrate, it ispossible to “self-solubilise” by warming the material to about 60° C. atwhich point the cobalt nitrate dissolves in its water ofcrystallisation. Preferably the impregnation and drying are repeateduntil the cobalt content of the calcined catalyst precursor is in therange 15-30% by weight.

The precious metal promoter is also included in the catalyst precursorby impregnation, using suitable soluble compounds such as the nitrate,which includes nitrosyl-nitrate, chloride, acetate, or mixtures ofthese. Preferably the precious metal promoter compound is a compound ofPt, Pd, Re, Ru, Ir or Au, and the impregnation is repeated until theprecious metal content of the dried catalyst precursor is in the rangein the range 0.05 to 0.5% by weight, preferably 0.05 to 0.2% by weight.In a preferred embodiment, the precious metal compound is a Ru compound.Ruthenium acetate, preferably ruthenium nitrosyl nitrate may be used.

The cobalt compound and precious metal compound may be impregnatedsimultaneously or sequentially. Hence, the promoter may be included inthe catalyst precursor before or after the cobalt, or at the same timeby combining the cobalt and promoter compounds in the same impregnatingsolution. Simultaneous co-impregnation of cobalt and precious metalpromoter has been found to work particularly well in the presentinvention.

The drying step may be performed, as for the magnesium-modified support,at 20-120° C. in air or under an inert gas such as nitrogen, or in avacuum oven. Again, the catalyst precursor may be dried to removesolvent prior to the second calcination, or the second calcination usedto both dry and convert the cobalt compounds to the oxidic form. Priorto the second calcination, the catalyst precursor may be pre-calcined atlower temperature, particularly after a first cobalt impregnation inadvance of a second or further cobalt impregnation. Such pre-calcinationis preferably performed by raising the temperature following the dryingstep to temperatures in the range 200-300° C. over periods of between 1and 6 hours. The second calcination may be performed in air or inertgas, at a temperature in the range 250 to 650° C., preferably 450 to650° C., more preferably 450-550° C. The calcination time is preferably≦24 hours, more preferably ≦16 hours, most preferably ≦8 hours,especially ≦6 hours. Typically the second calcination may be performedby increasing the temperature over a period of 1-6 hours to a maximumtemperature and holding there for a period up to about 6 hours.

To render the catalyst precursor catalytically active forFischer-Tropsch reaction, at least a portion of the cobalt oxide may bereduced to cobalt metal. The reducing step may be performed with areducing gas selected from hydrogen, synthesis gas or a mixture ofhydrogen and/or carbon monoxide with nitrogen or other inert gas.Preferred reducing gas streams that may be used include hydrogen- and/orcarbon monoxide-containing gases. Reduction is preferably performedusing hydrogen-containing gases at elevated temperature. Preferably thereducing gas stream comprises hydrogen at >25% vol, more preferably >50%vol, most preferably >75% vol, especially >90% vol hydrogen. Thetemperature of the reducing gas stream, and hence the catalystprecursor, during the reduction stage is preferably in the range 350 to500° C. The reduction time is preferably ≦24 hours, more preferably ≦16hours, most preferably ≦8 hours, especially ≦6 hours, with a minimumreduction time of about 2 hours.

Preferably at least 60% of the cobalt is reduced, i.e. the degree ofreduction (DOR) is preferably ≧60%, more preferably ≧75%, especially≧80%. A temperature-programmed reduction (TPR) method for estimating DORmay be used as follows:

1. Steadily increase the sample temperature to the desired reductiontemperature at 10° C./min, hold at that temperature for seven hours(TPR1).2. Without cooling back to room temperature, increase the sampletemperature to 1000° C. at 10° C./min and hold at 1000° C. for tenminutes. (TPR2). This gives complete reduction of all cobalt.3. Integrate the hydrogen uptakes from TPRs 1 and 2. The ratioTPR1/(TPR1+TPR2) is the degree of reduction (expressed as %).

The reduction may be performed at ambient pressure or increasedpressure, i.e. the pressure of the reducing gas may suitably be from1-50 bar abs, preferably 1-20 bar abs, more preferably 1-10 bar abs.

The gas-hourly-space velocity (GHSV) for the reducing gas stream may bein the range 100-25000 hr⁻¹, preferably 1000-15000 hr⁻¹.

Before the reduction step, the dried or calcined catalyst precursor may,if desired, be formed into shaped units suitable for the process forwhich the catalyst is intended, using methods known to those skilled inthe art. The shaped units may be agglomerates, pellets or extrudates,which may be spheres, cylinders, rings, or multi-holed pellets, whichmay be multi-lobed or fluted, e.g. of cloverleaf cross-section.

Following reduction, due to the reactivity of the cobalt metal to oxygenin air, the process for preparing the catalyst preferably furthercomprises a step of encapsulating the reduced catalyst in a hydrocarbonwax.

The catalysts may be used for the Fischer-Tropsch synthesis ofhydrocarbons. The Fischer-Tropsch synthesis of hydrocarbons with cobaltcatalysts is well established. The Fischer-Tropsch synthesis converts asynthesis gas mixture comprising carbon monoxide and hydrogen tohydrocarbons. The synthesis gas typically has a hydrogen: carbonmonoxide ratio in the range 1.6 to 3.0:1, preferably 1.8 to 2.2:1. Thereaction may be performed in a continuous or batch process using one ormore fixed bed reactors, micro-channel reactors (i.e. reactors in whichthe catalyst is disposed in channels typically with a cross-sectionalarea <150 mm²), conventional stirred slurry-phase reactors, jet-loopreactors, bubble-column reactors, or fluidised bed reactors. The processmay be operated at pressures in the range 10 to 60 bar abs andtemperatures in the range 150-260° C. The gas-hourly-space velocity(GHSV) for continuous operation may be in the range 100-25000 hr⁻¹. Apreferred operating range is 1000-20000 hr⁻¹.

The catalysts of the present invention have shown enhanced stability athigh operating temperatures, especially temperatures in the range200-260° C., particularly temperatures in the range 230-260° C. Fortemperatures in the range 230-250° the process may be operated at a COconversion above 40%, with a C5+ hydrocarbon selectivity ≧80% and amethane selectivity ≦15% preferably ≦10%. This selectivity stability,i.e. a C5+ hydrocarbon selectivity stability and methane selectivitystability, has been demonstrated for 24 hours or longer. Correspondingcatalysts without the magnesia promotion, or with higher levels ofmagnesium do not show the same selectivity stability. Conversionstabilities, defined as the initial CO conversion (before operating≧210° C.) divided by the final CO conversion (after operating ≧210° C.,particularly 230° C.-260° C.) under the same flowrate, temperature andpressure conditions, above 0.8 and even above 0.9 may be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by reference to thefollowing Examples and by reference to FIGS. 1, 2 and 3, wherein:

FIG. 1 depicts the comparative catalyst C performance vs. time; and

FIGS. 2 and 3 depict Example 1b performance vs. time.

DETAILED DESCRIPTION OF THE INVENTION Example 1 Preparation of CatalystPrecursors

a) Support.

A commercially available transition alumina powder (Puralox 100/150) wasimpregnated using an incipient wetness method with an aqueous solutionof magnesium nitrate hexahydrate (MgNHH). The damp material was thenspread onto a stainless steel tray, and calcined in air at 550° C. for 5hours with a 5° C./min ramp rate. Three modified supports were made,nominally with 0.5% wt Mg, 3% wt Mg and, for comparison, 6% wt Mg.

The amounts used were:

Example 1a: 0.5% Mg−39.6 g Puralox 100/150, 2.7 g MgNHH, 42.5 mLdeionised water.

Example 1b: 3% Mg−38.5 g Puralox 100/150, 15.9 g MgNHH 32.5 mL ofdeionised water.

Comparative A: 6% Mg−36.9 g Puralox 100/150, 31.6 g MgNHH, 20.8 mLdeionised water.

b) Catalyst Precursors.

All three modified supports were co-impregnated with aqueous solutionsof cobalt nitrate hexahydrate (CoNHH) and ruthenium nitrosyl nitrate(RuNN) by incipient wetness to produce catalyst precursors nominallywith 20% wt Co and 0.1% wt Ru. The amounts used were 49.50 g of CoNHH,0.16 g of RuNN and an appropriate amount of deionised water (14.0 mL,6.0 mL, and 2.5 mL respectively for the 0.5% wt Mg, 3%wt Mg and 6% wt MGmaterials). The damp materials were spread on a stainless steel tray andcalcined at 250° C. for 8 hours with a 5° C./min ramp rate. In addition,a comparison material, Comparative B, containing nominally 20% Co and0.1% Ru on the Puralox 100/150 was produced by an identical method.

Elemental Analysis by XRF on the three Mg-containing materials was asfollows;

Sample Co Al Mg Ru Example 1a 17.18 36.86 0.26 0.06 Example 1b 19.5833.28 2.06 0.06 Comparative A 18.26 31.07 4.13 0.05Cobalt surface areas were determined using hydrogen chemisorption asdescribed above.

Co Surface Area Sample name (m²g⁻¹ catalyst precursor) Example 1a 13.0Example 1b 14.7 Comparative A 12.9 Comparative B 11.2

Temperature-programmed reduction (TPR) performed on the catalystprecursors showed two main peaks in all the graphs, which correspond toCo₃O₄—to CoO and CoO—o Co metal transitions. The maximum of both peaksshifts to higher temperature with increasing Mg content, and there isthe appearance of a low temperature shoulder on the Co₃O₄—CoO peak.There is also an increase in the areas under the peaks with increasingMg loading, especially up to 3% Mg loading, indicating that the Mgmodified materials have a larger amount of reducible cobalt, althoughfor the higher Mg loadings this cobalt is only reduced at temperaturesabove 500 ° C.

1^(st) peak 2^(nd) peak Area Maximum Area Maximum Sample (a.u.) (° C.)(a.u.) (° C.) Example 1a 0.021 241 0.048 449 Example 1b 0.028 257 0.061468 Comparative A 0.033 267 0.060 498 Comparative B 0.015 240 0.034 405

Example 2 Catalyst testing

a) Varied GHSV.

Catalysts were tested by placing about 0.134 g of each precursor withina cell of a micro-reactor test unit. The catalysts were reduced at 425°C. in a H₂ and Ar flow, and then the temperature was reduced to 160° C.At this point CO was introduced to form a syngas mixture with a H₂:COratio of 2:1, the pressure was set at 20 bar, and then the temperaturewas raised gradually using a ramp of 0.1° C./min to 210° C. Thecatalytic test was started using a flow rate of 30 ml_(N)/min syngasthrough each cell. The temperatures were increased from 210° C. to 230°C. and 240° C. and the syngas flowrate adjusted at 230° C. and 240° C.to obtain a conversion above 40% at 230° C. The summaries of theexperimental results obtained are presented in the following tables. Thevalues of conversion, Selectivity to CH₄ and Selectivity to C₅+presented in these tables are obtained by averaging the experimentalvalues over the stated interval. A conversion stability was calculatedby dividing the final conversion by the initial conversion under thesame conditions.

Example 1a

Flow S S GHSV/ Temp/ rate/ Conv/ CH₄/ C₅₊/ Pressure/ Interval/ (ml/ ° C.ml/min % % % bar h (g_(cat)*h) 210 30 17.4 8.35 79.94 20 103-120 13373230 30 70.6 3.95 90.86 20 129-144 13373 230 50 40.88 7.07 84.85 20147-167 22288 240 60 62.88 6.88 86.4 20 196-219 26746 210 30 14.59 9.0478.59 20 237-271 13373Conversion stability=0.84

Example 1b

Flow S S GHSV/ Temp/ rate/ Conv/ CH₄/ C₅₊/ Pressure/ Interval/ (ml/ ° C.ml/min % % % bar h (g_(cat)*h) 210 30 11.94 9.21 70.44 20  80-118 13383230 30 60.16 4.21 87.75 20 132-140 13383 230 45 41.93 6.16 83.95 20151-166 20074 240 60 63.84 6.92 85.84 20 188-215 26766 210 30 11.79 8.1674.64 20 241-271 13383Conversion stability=0.99

Comparative A

Flow rate/ S S GHSV/ Temp/ ml/ Conv/ CH₄/ C₅₊/ Pressure/ Interval/ (ml/° C. min % % % bar h (g_(cat)*h) 210 30 0 N/A N/A 20 110-121 13483 23030 10.67 15.78 43.9 20 151-166 13483 240 60 25.13 8.72 74.47 20 200-21826966 210 30 0 N/A N/A 20 237-271 13483Conversion stability=0

Comparative B

Flow GHSV/ Temp/ rate/ Conv/ S CH₄/ S C₅₊/ Pressure/ Interval/ (ml/ ° C.ml/min % % % bar h (g_(cat)*h) 210 30 26.27 8.81 81.33 20  99-118 13363230 30 79.77 4.2 90.69 20 125-136 13363 230 60 40.56 7.71 84.26 20155-167 26726 240 60 71.55 7.39 84.23 20 192-215 26726 210 30 18.26 7.9583.24 20 237-271 13363Conversion stability=0.68

The main findings for these experiments are summarized as follows: Incomparison with Comparative Example B, the addition of Mg to the Co-Rucatalyst reduces the activity of the catalyst, especially at 210° C. Thehigher the amount of Mg added, the lower the activity of the catalystand Comparative A, with 4.12% Mg was ineffective at 210° C. However, theaddition of Mg in Examples 1a and 1 b leads to an increase in thestability of the catalyst. Returning to the start-up conditions (210° C.and 30 ml/minute feed) show that the catalyst doped with 0.5% Mg lostapproximately 3% of its initial activity, while the catalyst doped with3% Mg lost only 0.2% of its initial activity. Although the un-modifiedcatalyst is more active than the catalysts doped with Mg, after beingsubjected to testing conditions at 230° C. and 240° C. it loses 7% ofits initial activity and has a conversion stability of 0.68 compared toExamples 1a and 1b which have values of 0.84 and 0.99 respectively.

b) Fixed GHSV.

Further tests were performed in a micro-reactor using 0.1g catalyst, butunder a fixed GHSV of 16800 l/kg_(catalyst.)hour. Example 1b and afurther comparative catalyst were tested. Comparative C, likecomparative B, contained nominally 20% Co and 0.1% Ru, on a transitionalumina and was again prepared by impregnation. However, the rutheniumcompound used to prepare the catalyst was ruthenium acetate and thecommercial transition alumina used was HP14/150. Reduction of bothcatalysts was carried out at 380° C. using pure hydrogen over 7 hrs.Testing was performed at a H2/CO ratio of 2:1 at 20 bar and a GHSV of16800 l kgcat⁻¹.hr⁻¹. The results are given in the following table andare depicted in FIGS. 1, 2 and 3.

Sample Temp/ % CO Selectivity/% Code ° C. Conv CH₄ CO₂ C₂₋₄ C₅₊Comparative C 210 9.1 9.7 0.0 8.5 81.9 Example 1b 210 9.7 7.8 0.1 10.082.1 Comparative C 220 16.2 10.6 0.0 9.2 80.2 Example 1b 220 17.0 8.10.1 9.9 81.9 Comparative C 230 25.1 11.9 0.3 9.8 78.0 Example 1b 23028.4 8.7 0.2 10.4 80.7 Comparative C 240 33.5 14.7 0.5 11.3 73.5 Example1b 240 51.8 8.1 0.4 10.0 81.5 Comparative C 250 43.3 18.2 1.0 12.4 68.4Example 1b 250 67.6 9.4 0.9 9.3 81.4

FIG. 1 depicts comparative catalyst C performance vs time. The resultsindicate that the Comparative catalyst C offered conversion above 40%only at 250° C. where it rapidly deactivates. Deactivation is also seenin FIG. 1 at 240° C. and 230° C. The initial CH₄ level of 10 increasedas the temperature was increased, showing a falloff in selectivity, andremained at about 15% when the initial operating conditions (210° C.)were re-established. Furthermore the CO conversion had dropped fromabout 9% initially to about 4% after the operation at 250° C. i.e. aconversion stability in this case of about 0.44.

FIGS. 2 and 3 depict Example 1b performance vs time. In FIG. 2, Example1b demonstrated conversion above 40% at 240° C. and CO conversions at230° C. and 240° C. were stable. FIG. 2 also shows that Methane levelsbelow 10% were maintained throughout the test. The test was stopped at100 hours. A repeat test was performed and is depicted in FIG. 3. Inthis case, the process was heated directly from 210° C. to 240° C. andthen 250° C. before re-establishing operation at 210° C. The COconversion at 240° C. of about 50% was again observed, with operation at250° C. giving CO conversion at 65-70%. The CO conversion uponre-establishing the initial operating condition remained at about 9% andthe methane selectivity did not increase above 10%. Thus this test showsthat the magnesium-doped catalyst maintains its initial performance.

1. A process for Fischer-Tropsch synthesis of hydrocarbons comprisingthe step of contacting in a Fischer-Tropsch reactor a synthesis gasmixture comprising hydrogen and carbon monoxide with a catalyst preparedby reducing a catalyst precursor comprising 10 to 40% by weight ofcobalt oxide crystallites and 0.05 to 0.5% by weight of a precious metalpromoter, dispersed over the surface of a modified catalyst supportformed from a porous transition alumina the surface of which has beenmodified by inclusion of 0.25 to 3.5% wt magnesium.
 2. The processaccording to claim 1 wherein the synthesis gas mixture has ahydrogen:carbon monoxide ratio in the range 1.6:1 to 3.0:1.
 3. Theprocess according to claim 1 wherein the process is operated at apressure in the range 10 to 60 bar abs and a temperature in the range210-260° C.
 4. The process according to claim 3 wherein the process isoperated at a temperature in the range 230-250° C. at a CO conversionabove 40%, with a C5+ hydrocarbon selectivity ≧80% and a methaneselectivity ≦15%.